Differential pressure measuring devices usually include one of two design variations. In a first design two half bridge circuits are connected together to form a Wheatstone bridge. The half bridges are normally provided on a silicon sensing die. In this case, there are two separate dies, with a half bridge on each. Each of the devices are ported to the main or reference pressure. One device is ported to the reference pressure. For a differential pressure measurement, the half bridges are electrically connected to electrically subtract the high or main pressure from the low or reference pressure resulting in a voltage proportional to the difference pressure. These techniques are well known. See for example U.S. Pat. No. 6,612,179 issued on Sep. 2, 2003 to A. D. Kurtz and assigned to the assignee herein, namely Kulite Semiconductor Products and entitled Method and Apparatus for the Determination of Absolute Pressure and Differential Pressure Therefrom. This patent describes the combination of absolute and differential pressure sensing devices including a plurality of absolute pressure transducers, each transducer including a plurality of half bridge piezoresistive structures and a device which selectively couples at least one of the half bridges to another half bridge.
In other prior art configurations, a single pressure sensing capsule is employed with the reference pressure ported to the rear side of the silicon sensing die. The main pressure is ported to the top side of the silicon sensing die. This design requires the use of a Wheatstone bridge on a single die. The difference of the main and reference pressure results in the differential pressure. Again, the differential pressure results in a voltage output. This design requires the reference tube to be connected to the reference pressure inlet. In any event, in actual operation both types of differential pressure measuring devices can be subjected to pump ripple, or a sinusoidally varying pressure fluctuation. Normally pump pressure is desirable to be measured in systems having pumps. For example, in an automobile, the oil that lubricates the engine of a car must be forced at high pressure around channels in the engine. In order to operate, a pump is used, which pump normally is referred to as a gear pump.
The rotating cam shaft of the engine normally powers the oil pump, driving a shaft that turns a pair of intermeshing gear wheels inside a close fitting chamber. The oil enters the pump where it is trapped by the wheels. The wheels carry the oil around to the outlet, where the teeth come together as they intermesh. This action squeezes the oil and raises its pressure as it is close to the outlet. The speed of pumping is directly linked to the speed of the engine. In any event, in such a pump, the pressure at the output as well as pressure at the input is normally monitored. The pressures are monitored by a pressure transducer. However, these pressure transducers can be subjected to pump ripple or a sinusoidally varying pressure fluctuation. In an adverse situation, the pipe and cavity of the reference or main side of the sensor can be tuned to the frequency of the pump ripple. By this occurring, one creates a resonance in the tube which results in an amplified pressure being applied to the transducer. This amplified pressure can seriously harm the transducer as will be further explained.
It is also known that the pump ripple is a function of the number of gear teeth in the pump and the number of revolutions per minute of the teeth. This, as indicated, can vary as the RPM of the pump can vary, and hence such a tube must be selected to filter the range of frequencies to prevent resonance and amplification in the pump operating RPM range. It is understood that the resonance and amplification of the pump ripple pressure can exceed the rating of the sensing die or pressure capability of the structure. Exceeding the rated pressure imparts excessive stress on the die which experiences brittle failure. Aside from loss of the signal from the sensor, on a filter application, contaminates from the dirty side of the filter can be passed to the clean side, thus further destroying the sensor or equipment downstream. One therefore requires a pressure transducer which will operate to eliminate pump ripple or to eliminate varying pressure fluctuations in a sensor and still enable the sensor to be small and compact.